1. Field of the Invention
The present invention relates to an electrophotographic toner suitable for adding an inorganic external additive, and a manufacturing method thereof.
2. Description of Related Art
Generally, an external additive is added to an electrophotographic toner to provide flowability and the like. Examples of the external additive include such inorganic external additives as silicon dioxide (silica) having a primary particle size of about 5 to 20 nm.
However, when a toner stagnates in a developing machine during a long period of printing, the problem is that an (inorganic) external additive is buried in the toner because of a stress by agitation in the machine. To solve this problem, inorganic fine powder such as silica and titanium oxide having a particle size of 20 to 300 nm or more is combined as a so-called external additive having large particle size.
Meanwhile, Japanese Patent No. 3141783 mentions that wax components existing on the surface of a toner (particularly, pulverized toner) cause toner flowability to be worsened and an external additive to be buried, and proposes a capsule structure toner by particle aggregation method to solve this problem.
Japanese Unexamined Patent Publication No. 2000-147829 proposes a capsule toner wherein a shell is composed of resin having a higher glass transition temperature (Tg) than that of resin constituting a core material.
However, while the use of the above-mentioned external additive having large particle size and the methods described in Japanese Patent No. 3141783 and Japanese Unexamined Patent Publication No. 2000-147829 have some effect of inhibiting an external additive from being buried, the effect is not enough. Therefore, in case of printing for a long period of time or continuous printing at a low coverage rate, it is inevitable that an external additive is buried, causing problems such as deterioration of developing performance (decrease in ID), reduction in transfer efficiency, blocking of a developer and toner aggregation.